This invention relates to apparatus for controlling the current flow through first and second loads and, more particularly, to the use of first and second power amplifiers for controlling the current flow through first and second motors to thereby control the direction and speed of the motors.
There are, of course, many different types of power amplifiers used to control the current flow through a load. In the case where the load is a d-c servo motor, for example, it has been found advantageous to use a switching power amplifier of the bridge type. In bridge type switching power amplifiers, diagonally opposed power switching elements are caused to conduct current through the motor in a given direction with one set of diagonally opposed elements causing motor current flow in a forward direction, and the remaining set of diagonally opposed elements allowing motor current flow in the reverse direction. By controlling the conducting intervals of the sets of diagonally opposed switching elements, current flow through the motor can be proportionately controlled.
Bridge type switching power amplifiers may be of the pulse width modulation (PWM) variety. In such power amplifiers, a triangle wave generator is normally provided for generating a trangular wave oscillator signal that is summed with a servo error signal to provide a pulse error signal. The pulse error signal in turn is operated upon to generate control signals for controlling the switching operations of a pair of switching elements, i.e., either the lower opposing pair or the upper opposing pair. Exemplary PWM switching power amplifiers of the above type are disclosed in U.S. Pat. Nos. 3,422,326, 3,471,759 and 3,525,029.
Many machines and systems now exist containing two or more movable elements, each driven by a separate motor individually controlled by a servo control system and associated power amplifier. Serial printers which include a rotatable print wheel mounted to a linearly movable carriage are an example of such machines. Movement of the print wheel and carriage are controlled by separate d-c servo motors driven by respective servo control systems and power amplifiers. The power amplifiers currently employed in such printers, although bridge type switching power amplifiers, do not employ PWM techniques. Rather, they are of the so-called "free running" type. An example of this type of power amplifier is disclosed in now abandoned U.S. Application No. 684,522 filed on May 10, 1976 in the names of Frank D. Ruble, et al for CONSTANT CURRENT POWER AMPLIFIER and assigned to the assignee of the present invention.
One problem with servo controlled machines employing multiple free-running switching power amplifiers is the presence of audible noise occasioned by the natural oscillating frequencies of the servo motors when maintaining constant speed levels. These frequencies may be as low as 1 KHz, well within the audible range. Another cause of audible noise is the beat frequencies developed as a result of the differences in frequency and phase among the natural oscillating frequencies of the multiple servo motors.
One way to reduce audible noise is to employ the use of a PWM switching power amplifier of the type disclosed in U.S. Pat. No. 3,525,029 to drive each servo motor. Each power amplifier would then have its own oscillator, such as a triangle wave generator, to generate an oscillator signal of a desired frequency of oscillation to control oscillation of the respective servo motor during constant speed maintenance. By setting the frequency of oscillation at a value above the audible range, e.g., 23 KHz, audible noise attributable to the oscillation frequencies themselves would be eliminated. However, this procedure would not eliminate the audible noise attributable to beat frequencies, since it would be extremely difficult to design completely balanced triangle wave generators for each power amplifier whose output oscillator signals would be constantly synchronized in phase and at the identical frequency.
Accordingly, it would be desirable to provide an apparatus for controlling the current flow through a plurality of loads, such as servo motors, wherein audible noise attributable to the oscillating frequencies of the servo motors during constant speed maintenance is substantially eliminated.